1. Field of the Invention
The present invention relates to a method and apparatus for measuring a flicker level, and more particularly, to a method and apparatus for measuring a level of screen flicker visible to the human eye.
2. Description of the Prior Art
Liquid crystal displays (LCSs) are increasingly being used for the display device in televisions, personal computers, etc., and in many state-of-the-art equipment such as automotive navigation systems and simulation devices. LCDs are significantly lighter in weight and slimmer, consume far less energy and can reproduce a wider range of colors than any competing technologies.
LCDs apply an electric field to liquid crystal material having an anisotropic dielectricity and injected between two substrates, an array substrate and a counter substrate, arranged substantially parallel to one another with a predetermined gap therebetween, and control the amount of light permeating the substrates by controlling an intensity of the electric field to obtain a desired image signal.
Formed on the array substrate are a plurality of gate lines disposed parallel to one another, and a plurality of data lines insulated from and crossing the gate lines. A plurality of pixel electrodes are formed corresponding to respective regions defined by the intersecting data lines and gate lines. Further, a thin film transistor (TFT) is provided near each of the intersections of the gate lines and the data lines. Each pixel electrode is connected to a data line via a corresponding TFT, the TFT serving as a switching device therebetween.
Each TFT has a gate electrode, a drain electrode, and a source electrode, and the pixel electrodes are connected to the drain electrodes. Here, common electrodes are disposed on either the array substrate or the counter substrate.
The electric field applied to the liquid crystal material is generated by a difference in levels of common voltage and data voltage applied respectively to the common electrodes and the pixel electrodes provided in the LCD. An intensity of the electric field is controlled by changing data voltage or common voltage levels.
As the liquid crystal material degrades if the electric field is applied to the liquid crystal material continuously in the same direction, the direction in which the electric field is applied must be constantly changed. Namely, a value of the data voltage minus the common voltage must be repeatedly alternated from a positive value (hereinafter referred to as positive voltage) to a negative value (hereinafter referred to as negative voltage).
Such a switching of electrode voltage values between positive and negative values is referred to as inversion drive. Among the different types of inversion drive methods are frame inversion, line inversion, dot inversion, and column inversion methods.
In frame inversion, in which the polarity of data voltage is inverted to frame cycles (typically 60 Hz), positive voltage is applied in odd frames, while negative voltage is applied in even frames. Here, it is established such that a root mean square (RMS) of the positive voltage is the same as a RMS of the negative voltage.
However, in the actual performing of inversion drive in the LCD, kickback voltage is generated by parasitic capacitance in the pixels such that the RMS of the positive voltage comes to differ from the RMS of the negative voltage. Accordingly, a brightness of light permeating the liquid crystal material in the odd frames and that of light permeating the liquid crystal material in the even frames become dissimilar. This results in screen flickers generating in units of one-half of frame frequency of 60 Hz, or 30 Hz.
Such a screen flicker is measured using Formula 1 below introduced by the Apple Corporation ##EQU1##
In the above Formula 1, F is the flicker level, and Po and Pf are amplitudes respectively of DC elements and AC elements (flicker elements) of light emitted from the LCD panel. Namely, according to the prior art flicker level measuring method, the level of screen flicker is the ratio of an amplitude of flicker elements to DC elements of light.
Referring to FIG. 1, shown is a graph illustrating Po and Pf of light emitted from an LCD panel. In the drawing, a brightness of light is realized by a sine function related to time, an average value (DC elements) of the sine function being Po, and an amplitude of the sine function being Pf. In FIG. 1, as Po is always larger than Pf, the flicker level attained using Formula 1 is always a negative value.
According to Formula 1, the flicker level is determined with considerations of merely the brightness of the light (DC elements and AC elements) emitted from the LCD panel, but other factors besides the brightness of the light such as screen size, distance between the screen and user, involuntary adjustment of the size of the pupil, etc. also determine the amount of screen flicker visible to the human eye. Accordingly, the flicker level attained using Formula 1 does not take into account these other factors.
Reasons why the flicker level attained using Formula 1 and the flicker level visible to the human eye are different will be explained hereinafter.
In Table 1 below, shown are DC elements Po and flicker elements Pf, and various flicker levels attained using Formula 1 when the difference in common voltage and data voltage is applied to 64 gray levels. A graph of the flicker levels according to gray levels of Table 1 is shown in FIG. 2.
TABLE 1 ______________________________________ Flicker elements Gray level DC elements (Po) (Pf) Flicker level (F) ______________________________________ 1 0.83 0.01 -23.86 5 0.88 0.01 -22.03 9 1.01 0.02 -17.63 13 1.24 0.04 -14.87 17 1.83 0.05 -15.32 21 2.60 0.08 -15.35 25 3.88 0.11 -15.58 29 5.40 0.14 -15.85 33 7.52 0.19 -16.06 37 9.77 0.19 -17.02 41 12.55 0.17 -18.60 45 15.74 0.19 -19.18 49 19.35 0.16 -20.70 53 24.12 0.16 -21.85 57 28.80 0.06 -26.63 61 33.91 0.01 -38.02 64 34.84 0.01 -34.73 ______________________________________
According to Table 1 and FIG. 2, a flicker level value attained using Formula 1 at a gray level of 13 is largest. However, in actuality, a flicker level visible to the human eye is largest at a more medium gray level of 32. Reasons for this will be explained hereinafter with reference to FIG. 3.
FIG. 3 is a graph illustrating transmissivity of light with regard to voltage Va applied to liquid crystal material of the LCD. In the drawing, light begins to transmit through liquid crystal material when the voltage Va applied to the same is above a threshold voltage Vth, with the transmissivity of light increasing as the voltage Va is increased. However, when the voltage Va exceeds a saturation voltage Vsat, the transmissivity of light no longer increases.
When the voltage Va applied to the liquid crystal material is at a point roughly in the middle level between the threshold voltage Vth and the saturation voltage Vsat, the transmissivity of light is greatly affected by even slight fluctuations in the voltage Va. Namely, small changes in the voltage Va in the middle level between the threshold voltage Vth and the saturation voltage Vsat produce large differences in light transmissivity. Accordingly, flickering is most visible to the human eye in this central gray voltage level.
Therefore, the prior method of calculating flicker levels is not accurate.